The present relationship between hearing, vocal learning, and vocal development in a small Australian parrot - the parakeet or budgerigar (Melopsittacus undulatus). While similar in some respects to the more familiar songbirds, vocal learning in budgerigars may also be quite different in significant ways. This is part of the focus of the present proposal. We know already that adult budgerigars have the ability to learn new calls very quickly with a capacity that appears unlimited. Auditory perceptual learning plays a central role in determining the characteristics of the species vocal repertoire. Furthermore, social and visual cues are of paramount importance. From recent cross-modal perceptual experiments, it appears that budgerigars may provide a unique system for examining how acoustic and visual information is coordinated in vocal learning. The proposed experiments will characterize: (1) the nature of perceptual categories for species-specific vocal signals in this species, (2) how the specialized auditory perceptual system of budgerigars is matched to the extraordinary capacity for vocal learning, and (3) what social and visual cues are coordinated to guide the development this learned vocal repertoire. These experiments are aimed at ultimately discovering the general biological principles which are capable of organizing and maintaining a complex, learned vocal communication system. Some of these general biological principles are clearly involved in the development and maintenance of complex, learned behavior in humans - the most relevant instance in the present case being language acquisition. The comparative behavioral approach espoused in this proposal - proceeding as it does with parallel neuroanatomical investigations - offers an exciting opportunity to understand the biological foundations of vertebrate learning.